avformat/nutenc: change check to match comment

The code appears to assume STABLE will never change
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutenc.c

@@ -410,7 +410,7 @@ static void write_mainheader(NUTContext *nut, AVIOContext *bc)
         avio_write(bc, nut->header[i], nut->header_len[i]);
     }
     // flags had been effectively introduced in version 4
-    if (nut->version > NUT_STABLE_VERSION)
+    if (nut->version > 3)
         ff_put_v(bc, nut->flags);
 }